One thing I love about that particular one is that you can't see the contents of the last cell before you read it, as it is a sub story on the paper. Usually you can see a cell ahead from peripheral vision, particularly if it is a vivid illustration. Sort of softens the impact a little

I think it just causes the body to relax, and therefore empty it's bladder. It tends to work best when they're drunk though. Especially since you're more likely to need to piss and you're less likely to notice your hand being dipped in water.